Pulse-echo identifying system



April 15, 1952 ,F. c. WILLIAMS 7 2,592,777

PULSE-ECHO IDENTIFYING SYSTEM Filed Aug. 21', 1946 3 Sheets-Sheev l INTERROGATOR PULSE TRANSMITTER PULSE RESPONDER MODULATdR wuss cnzcurr T'MING SUPER-REG TRANS 'ENEKATIVE J E I 650. G szzcvzvALvs VALVE TIME I BASE I GENERATOR POWER x DETECTO SUPPLY PULSE MOTOR RE PULSE. eeumm CEWER AMPLIFIER PULSEWIVTH F I CONTROL g cuzcuw swEsP l SWEEP 2 SWEEP 3 SWEEP 4 l8 /s|Z /S,l8%---lZ"/s\|8 qs-l2%\l8/s 2$ i 7 0 M ---'N W Z F invenfor u C. WIL g I B REDERK. I LIRNS A ttorn ey April 1952 F. c. WILLIAMS PULSE-ECHO IDENTIFYING'SYSTEM 3 Sheets-Shaev 2 Filed Aug. 21, 1946 April 15, 1952 F. c. WILLIAMS PULSE-ECHO IDENTIFYING SYSTEM Filed Aug. 21, 1946 3 Sheets-Sheev 3 lNl/E/VTOR FREosmq ('1. MALLIAHS JATTORNEKS' Patented Apr. 15, 1952 UNITED STATES PATENT OFFICE PULSE-ECHO IDENTIFYING SYSTEM FredericlCalland .Williams, Great Malvern, England, assignorto Minister of Supply in His Majestys "Government of vthe United Kingdom of Great Britain and Northern Ireland; London,

England Application August-21, 1946, Serial-N0. 692,085

- Section 1, Public Law 690,'August"8,{1946- Patent expires March'13, 1963 vent anyunauthorised observer from. detecting i the source of the reply.

Theapparatus employed in these systems comprises a wireless transmitter fdr radiating a call-- Such systems ing=-signal located at "the observation'post and knownas the interrogator, and a-combined wire less-receiver and transmitter located "011121,: mobile craft or a: ground: station and known. as the: responcle'r which isautomatically triggered bythe calling orzinterroga'ting signal. andrthereupon radiates an identification: signal in reply thereto.

One object of the present invention :is to provide a -system ofthe kind referred to and apparatus therefor inwhich the means of recognition' is limited to one or more authorised observers provided with appropriate received signal in-* dicating and interpreting equipment and? in which a high" degree of :secrecy andureli'ability:

maybe obtained in operation.

*Anotheryobjectof the inventionxis to provide.

awir'elessasigmmlling system of the 'kind referred to inwhich" the interrogator is arranged .to radiate a series of separated radio-frequencypulses1 and in which the responder is arrangedlto radiate separate: counterpart pulses in "response to: each of certain individual ipulseszof the interrogation pulse series, selection o of those interrogation pulses to whichcounterpartradiation takes place being controllable at the responder "so as to "pro vide a characteristic coded response:

. A further object of the invention isgto'provide awireless signalling system ofzthe kind referred tov Whereinrthe interrogator is arranged :to radiate aseries: of separated Jradio +fr'equency pulses :and: inzawhich the: responder is arranged to radiate separate counterpart: radio frequencytpulses/tin response to -a chosen number of said .received interrogation pulses," the time *duration :of said counterpart pulses being variable; according .to. a; predetermined coding sequence so as to provide a characteristic identification signal.

Yetasfurther object of the invention is to pro vide a: responder .foriuse in a wireless;signallingo system of the kind referred to and comprising radio receiving means, radio transmitting means controlled "by the signal output of said receiving means-so as to effiect the transmission of counterpart radio frequency pulses inwresponse to individual interrogating signal pulses, and" means for repeatedly? imposing: a characteristicand predeterminedvariationin the-nature of the transmitted counterpart? pulses so. as "torprovide a characteristic :codedmesponse.

A particular formaof responder :embodying the invention comprises a.-superrregenerativereceivingvalve'which serves to trigger a pulsed or selfblockingpscillator; the :oscillator; :being: regeneratively backcoupled through: an: amplifier during the period of each pulserra'diated by the transmitter; The super-regenerative: receiving valve and the oscillator.v may have: a common envelope.

The receiver effecting: identification is," inoer.-

tain' forms of apparatus embodying .the- ,inven tion; operated in 1 combination: with any 'known:

radio-location vequipment ofthe: type: which uses a pulsed 'exploringswave to indicate the position of a distantbodyby'reflection: Since the reac-' tion of the responder to aninterrogatingimpulse is practically instantaneous; the time-interval, required for recognition at the observation post will be: substantially thesameg as that-which separates an outgoing exploring-pulse from its refle'ctedecho. .If'the interrogator ancta signal.

interpreter are both linked to' the'ztime-baseiof the cathode raytube indicator ofthe radio-location equipment, the; coincidence voitthe two'types? of. signal serves toidentify the character as well as'the position of any selected one of several bodies simultaneously within the field of exploration.

In applying the invention to the interrogation of'a'fi'xed beacon station, a voltage representing the time-interval between the .outgoingncall and the incoming response maybe: used to produce an indication in a: time-base synchronized "with the interrogator and. thus similarly'to indicate thedi'stance; aswell as the identity of the interrogated beacon.

In order that the-invention may be'more clearnal .andza numberiof examples'xof-s coded responses signals.

Figures 3 and 4 show alternative forms of responder circuit arrangements.

Referring first to Fig. 1, which illustrates in block schematic form, one typical signalling system according to the invention, the interrogator is constituted by a radio-location equipment comprising essentially a pulse radio transmitter feeding its output power to a suitable aerial and controlled in its pulse modulation by the output from a pulse modulator which is rigidly controlled in turn by a timing pulse generator operating at a suitable chosen frequency, i. e., that required for the pulse-recurrence rate. The timing pulse generator also controls the operation of a time-base generator which is associated with a cathode-ray display tube, the cathode beam of which is also arranged to display by its deflection at right angles to that of the time base, signals received by a pulse receiver also timed to the transmission frequency and coupled to a suitable receiving aerial. The arrangements depicted schematically are of well known form and need not be described in detail, the

operation broadly being the regularly repeatedtransmission of powerful pulses of radio energy at a chosen radiation frequency from the transmitter accompanied by a synchronized sweeping movement of the cathode beam of the associated cathode ray tube in its time-base defiection across the tube screen. Any returned echo signal is picked up by the receiver aerial and, after suitable amplification and detection, effects deflection of the tube beam at a position displaced from the commencement of the time= base sweep by an amount which is a measure of the time-delay thereof after the outgoing pulse which caused it and which is, therefore, indicative of the range of the reflecting object.

The distant object, in the example shown, is provided with a responder according to the in- Vention for establishing its identity; This responder comprises an aerial connected to a tuned oscillatory circuit which is common to the input of a superregenerative receiving valve and to the output of a transmitting oscillator valve which latter, however, is held in suppressed condition by the application of suitable negative gridbiasing potential. The tuned circuit is made variable in its resonance frequency and the latter is arranged to be swept repeatedly over a range of frequencies such as are utilized as the different radiation frequencies of a number of radio-location equipments. This sweeping is conveniently effected mechanically by motive power derived from the shaft of a motor generator serving to provide the requisite high-tension supply to the apparatus.

Upon resonance of the tuned circuit with the frequency of any interrogating or explorin pulses, these will be picked up and, after amplification by the super-regenerative receiving valve, demodulated in a detector and the extracted modulation envelope, in the form of a pulse, further amplified by a suitable pulse amplifier. Theoutput of the latter is then applied by way of a pulse-width control circuit comprising a suitable resistance/capacitance network, to the transmitting oscillator valve whereby the suppression of the latter is temporarily overcome for the duration of the pulse output from the control circuit and radiation effected by a much magnified response or counterpart pulse to the interrogating pulse which initiated it. This response pulse supplants the normal and relatively weak echo pulse produced by 4 reflection from the object in which the responder is located.

This response pulse travels back to the interrogator and is there received in similar manner to echo pulse signals, but by virtue of its greatly enhanced power, causes a recognizably amplified deflection of the display tube beam.

In order to permit identification of the particular object carrying the responder from other objects having similar responding facilities, the motor generator is also arranged to drive means which periodically, and repeatedly, effect some alteration in the character of the response signals, so as to provide a coded response instead of an indiscriminate response to any incident interrogation pulses. For example, such means may alter the characteristics of the pulse width control circuit whereby the response pulses are each made recognizably longer or shorter in their time duration and hence in the length of the time-base trace on the screen of the cathode ray tube of the interrogator receiver. Alternatively or additionally, such motor generator may control a power supply to the receiving and transmitting valves whereby the latter are pe-, riodically put out of action.

Fig. 2 attempts to illustrate in diagrammatic manner a number of positive coded response signals and shows also how these are each initiated by and only occur as a result of the incidence of an interrogation signal. At (11 i shown a continuous series of interrogating pulses radiated at a chosen repetition rate, say 500 per second, and at one radiation frequency, say 18 mc./s., from a first interrogator. Diagram (12 illustrates a similar series at a different repetition rate, say 200 per second, and at a different radiation frequency, say 12 mc./s. It will be appreciated that no attempt has been made to depict the pulses and the intervening time intervals to a common scale as this is impracticable since the individual pulses are usually of the order of 1 to 25 microseconds duration and the intervening intervals of the order of l to 40 milliseconds. Similarly the total period illustrated is assumed to be of the order of, say, 12 seconds, divided into four equal periods of 3 seconds each, marked sweep 1, sweep 2, sweep 3 and sweep 4. The tuned circuit of the responder is assumed to be swept through a tuning range of 18 mc./s. to 12 mc./s. once every 3 seconds.

In diagram 1) is shown one possible coded response, sent to each of the two interrogators. In the first 3 second sweep period response pulses n are radiated while the tuned circuit is resonant at or near the interrogation frequency of 18 mc./s., i. e., for a period of, say, one-tenth of a second, at the beginning of the sweep and further response pulses 12 while the tuned circuit is resonant at the other interrogation frequency of 12 mc./s., at the end of the sweep. This state of affairs is continued during the following two sweeps but in the fourth sweep of each cycle, the power supply is arranged to be cut-off whereby there is no response action whatever. The resultant display signal at the interrogator is accordingly a much amplified deflection lasting for about one-tenth second and repeated once every 3 seconds for three periods and then followed by a blank period, after which the whole cycle is repeated again and again for so long as the re-' form consisting of response pulses in the first two.:perio.ds and-..blank;. periods rorstheiremaining two:.of every cycle.:-.- it

Instead of oriinaaddition to the periodic. sup: pression .of 1 response. pulses: in: certain. 1 periods change -.:may.;-be.wmade; as described; in. ..the ;effective .width :.or::time.;duration1 of .the .response pulses. Diagrams id; 1eand-:1, illustratemxamples of..a. code..-built.-.-up upon-;this.;-width control-basis. In1:diagram-:d. each; response pulse in. each sweep periodis .of the 'sameswidth; e. g., Zjmicroseconds duration-or -narrow. In. diagram 12 the response pulses are .narrow during-the first 'two periods of: every.foureperiodrcyclebut appreciably wider, say of :8. microseconds duration;;.during ;.the:--re.-' maining two periods: Diagram. frshowsa-a further variation :wherein alternate :periods' :provideznare rowand-thenfwideiipulses.

' Referring now to: Fig1 3 :of 'the drawingspa simple form: of responder :is shown, suitablei'for co-operationuwith. one'or more radio-location stations each of i which transmits; a: pulsed ex-a ploring wave;on a:selected carrierefrequency. A

common aerial circuit L3, lC2ll;;:.coupled to "aerial terminals PI is connected. between. the: two anodes "a1 d2; of a double'ttriodevalve"V2. The grid-r91 of this valve is :regenerativelyt: backs cou-. pled to the. circuit. L3; CZtlwby.thezcondenser -C I 9 and is also-subjected to.quenching oscillations fromra valveuvl sorthat .the :tr iode section. of the valvecontrolled bythe gridxfgi. actssasz a: supergenera-tive receiver. circuit. The grid gz;:of:the other. triode .sectionitis; also regenerativelyt backs coupled :to :the circuit;:L3',..C20Zbykthejcondenser. Cl 8 whereby; this. second :trioda-section forms; an oscillator -t which: is; however, normally-1 held ine. operative -by reason: ;of grid .92: "being biased: to anode current cut-off. Theanode-az is also :back-. coupled to the .grid "92 .throughr-a. circuit. which includes acondenser CI 2 and a rectifier 5V3 which feeds the grid of an amplifier V4. Thewoutput from theanode of the latter. is fed: backiin part through a condenser .CIB'and resistances-R9; RI ii to the grid in oftthe valve1V2, in-partthrough a resistance .RI I i to the common-cathode. :of. the same valve-and inrpar't through a resistance R13 to a control circuit CC which is described in detail later.. The valve.VZtherefore-operates:as a sensitive relay in which the triodesectiontcomprising anode arand gridm;amplifies-@a calling signal and applies it through .the rectifier-V3 and amplifier V4,. to trigger. the. triodesection controlled by the gridtgz, which thereupon. rapidly builds. up persistent high-frequency oscillations which are automatically-squegged: These-oscillations are radiated, from :the common aerial-Pi as radio-frequency pulses of atimerduration p redetermined in amanner tohe described.

The condenser C20. of..the.aerial. coupling .cir.-. cult is 1 automatically-and: mechanically rotated tosweep the aerial tuning, say, .once.every three seconds, through asband-widthtof;-.s ay,.sixmega cycles. Each of the radio-location:-stationswith which theresponders co-operatespwill :transmit a pulse modulated exploring wave on one tor-. other ofv the sub-bandsawithin. .theisweepgofwthertuning fiedrsignal appearing acrossizthe pirouit L3,; C2 0. is applied;throughzcondenser: Cl 2 therectifier- V3 and the resultant- .negative. D. C.. potential fedto thecontrol .-grid 'of amplifier valve V4. The. amplified 1positive'f potential-irappearing eat :the anode of valve V4 r is: :applied; through: condenser C13, gridtgzzohvalvev I -so that-the suppressedttriode oscillator'circuit:controlled.;by@ this grid. is; triggered andzcontinuesto oscillatefor, a predetermined'rperiod; thus-producingacounterpart pulse of .signal; :energy; for. each received interrogation pulse. The feedeback-fromanode-aaz-to gridigz through the pulse namplifier V4 :;is maintained until. the rectified voltage .from the diode V3 blocks rathEyglfidxOf thexamplifier V4.

The period; of .eachcounterpart pulse is determineclikbythe-time: constants of the circuit, and morezparticularly; by the "value of the condenser Ci 3; which afteraperiod of .say six microseconds of oscillation of the triodeesectionaagz of valve V2, becomes charged by grid current sufiiciently to block :or .squegg the grid g2 and'so stop fur-. ther: oscillation. There follows aperiod offsay 200: microseconds during-:which the charge on condenser CH3; leaks away and the circuit is gradually-restored to the receivingcondition, this periodzbeing sufl'iciently long-to prevent any risk of interaction? or singing? between: this responder :and; another; similar responder within: comparatively;close rangeof each other;

The-response pulsed, oscillations 1 are suitably coded and-rconstitute the" identification. signal. The outgoing pulses {are-automatically keyed either-by;--a.emakeeand-breakimethod or :by systematically varying the normalwidth oftheradh ated,;;pulses so as to produce;- a;characteris.tic Morse.- pattern. on. a 1 receiver :cathode ray tube. With .-a-codingcycle sofxsay twelve seconds; made up;of:' four three-second tuning sweeps orperiods; make-and-breake keyinggimay be-used :to, produce say threei consecutive periods of narrow pulses followed by'aeblanl z period as shown-in diagram b';of--.:F'i ;2;' or two consecutive periods: of narrow pulses". followed by:v two": consecutive blank PEIiOClSHELSLShQWHT in diagram c-of :Fig. 2; and soon; Preferably1 the high-tension supply to the anodesof :the'zvalve V2 Z-isswitched onand 01T,: to give the required: coding "sequence by -means :of a relay which may be energised automatically by contactsone-rotating shaft; In ".the. variablewidth method of keying, one Morse character may consist of. four consecutive periods of narrow pulses as shown. in. diagramd of Fig. 2; another'may comprise .two consecutive periodsof narrow pulses followedby two consecutive periods of'wider-pulses .asshown in diagram re of Fig.2; athird may be formed ofalternate periods of narrow and .widepulses gas; shown in;. diagramif of Fig, ,2; and so on. Thezpulsesiare varied from narrow to wideby meansrof an electromagnetic relay or:switch S5 which will;be'-referredtto:;asta coding-switch and which inserts aicondenser .C I 4 in parallel with. the :condenser;Cl3, thereby in.- creasing the-time constant of the resistance/cae pacit network :and have the;.duration;. of each radiated pulse-from, say;six:to.1eighteen microe seconds.-;;,due ito the.- longer; period; required; to charge the parallel combination by the: grid current.

As shown, the coding. switch: 65.. is. energised from the 1 primary current'supply source. :which feeds amotor-generator Zwhich in turn provides the H; T. supply; voltages for pthepdevice and also controlsthe sweeprof; thetuning;condenser CZil: Primary current is;takenr.from:a:batterysthrough terminals P6 in the control circuit CC and is fed to the motor-generator Z through a variable resistance R8, a remote-control switch S2, and the contacts 3 and2 of a plug P2. The motor-generator Z drives a low-speed shaft Y which is arranged constantly to rotate the tuning condenser C20 through the allotted frequency band twice during each revolution, and also to operate two cam switches S3, S4. The switch S4 periodically makes and interrupts the high-tension supply to the oscillator valve V2 so that the latter has two live periods in each revolution of the shaft Y coincident with the tuning condenser C20 sweeping the tuning band from one end to the other and two dead periods while the condenser C20 is returning to its starting point. The switch S3 which forms part of the circuit controlling the coding switch S5 closes for half the period of each revolution of the shaft Y, i. e., during one of the two live periods of V2. The operation of the coding switch S5 is also controlled by a selector switch SI which has three positions. In position of switch SI the energizing circuit of coding switch S is permanently broken and it is therefore not operated; in position 1 the coding switch S5 is continuously energised; and in position 2 it is energised via the cam-switch S3. Accordingly when the selector switch SI is set to position 0, the signal pulses are consecutively narrow; in position 1 they are consecutively wide; whilst in position 2 they are alternatively wide and narrow, since the transmitter is energised twice per revolution of the shaft Y by the cam S4 and the coding switch S5 is only closed once per revolution by the cam S3.

A lead from the anode of the pulse-amplifier V4 is taken through a resistance RI3 and. contact 6 of the plug P2 to the control circuit CC which includes a monitoring meter MI and a headphone plug P4 to allow the operator to check the outgoing pulses. The control circuit CC also includes a variable resistance R! which is in series with the cathode resistance R6 of the valve V2 and is used to vary the sensitivity of reception within fine limits by control of the standing bias on the grid 91 to such a value that the positive peaks of the quench oscillations from valve VI just allow oscillation in the known super-regenerative manner.

It will be seen that an arrangement as shown in Fig. 3 will respond to the interrogating or exploring pulses, in due sequence, of any radiolocation transmitter that is operating at a frequency within the tuning range of the condenser C20. The resulting identification signals are picked up by the ordinary receiver of the radiolocation station, which acts as the interpreter, and they are caused to appear on the same timebase and in close proximity to the reflected echoes from the same craft. This automatic correlation informs an observer both of the position and of the character or type of any particular craft selected for scrutiny. Owing to its relatively wide frequency band width and greater amplitude the identification signal can be more clearly distinguished from the reflected echosignal if the receiving circuits are slightly detuned.

The responder may be modified for co-operation with different types of radio-location apparatus by arranging that all calls are made, and all responses are given, at frequencies which are well removed from those used for the pulsed exploring waves. In such a case it follows that each radio-location station must be provided with a separate pulsed interrogator and also with a separate receiver, or interpreter, both being tuned to the carrier-wave allotted to that particular station for identification signalling.

An alternative form of responder circuit is shown in Fig. 4 of the drawings in which tuning of the main oscillatory circuit comprising inductance L6 and variable condenser C9 is repeatedly swept once every three seconds through a frequency-band of 30 megacycles per second, for example, between 157 and 187 megacycles per second. Any incoming call signal is fed to that circuit from the common aerial socket P8 through a coupling coil L1. The received signal is then fed to a super-regenerative valve V2 to which quenching oscillations are fed from a quenching valve VI. The output from valve V2 serves to trigger an oscillator valve V3 which is normally biased to cut off in substantially similar manner to that previously described. In this case the resulting amplified input signal oscillations are fed to and rectified by a diode V4 and are then applied to the grid of an amplifier V5, which is coupled by way of an adjustable C, R network to a cathode follower valve V6 having a cathode load resistance RIG. The resulting positive voltage from the resistance RIB is fed back to the grid of the transmitter oscillator valve V3 which is also connected to the tuned circuit L6, C9 to cause the build-up of persistent oscillations in that circuit which will still be tuned to the frequency of the initiating received signal. In this embodiment the highfrequency oscillations generated by valve V3 persist for a period of time which is determined principally by the time constants of the coupling circuits between the valves V5 and V6, and. more particularly by the values ofthe condensers C20, CI9, the latter of which is controlled by a signal coding switch IA.

Owing to the presence of random noise and other variable factors in the super-regenerative circuit, the degree of build-up applied during each quenching cycle will, in general, be variable. In order to stabilize this efiect, a part of the output from the rectifier V4 is fed through a condenser C30 to the control grid of a valve V9, which includes in its anode circuit an ironcored transformer LII], LII tuned to the quenching frequency. The voltage across the winding LI 0 is rectified by a diode V8, and is applied from the load resistance R22 to the grid of a valve V'I, where it appears as a potential drop across the cathode resistance RI 8. From here it is fed through a resistance RH and choke L3 to the grid of the super-regenerative amplifier V2, and so automatically checks any increase in the general level of the oscillation in the tuned circuit L6, 09. The stabilising effect described forms no part of the present invention and is the subject of co-pending U. S. Serial No. 692,084, filed August 21, 1946.

Provision is also made to inhibit the responder circuits during the operation of any other radio transmitter that may be carried by the interrogated craft. For this purpose a paralyzing voltage, synchronized with the other transmitter, is applied through a socket P1 to a diode VII, and through a resistance R33 to the grid of a valve VIII. The resulting negative impulses are applied by the valve VII through a condenser C3I and resistance R28 to the grid of the valve V2, and through a condenser C33 and resistance R29 to the grid of the quenching valve VI, and throw both these valves out of action. Simulpage 'tanecusly a 'posi-tive voltageis ied from the'cathode resistance-R30 of fthe valve :V2I through: a condenser' CM 'to ithetcathodeiofrthe frectifier .V l to ren'derthat valve :non-conductive. .-.As. :a' result the responderiisttemporarily: rendered; insensitive to iall.v signals; includings'thosei from the .localztransmitter.

The.;aerial.'tuningmondenserfle.is driven: from a .imotori-generator through; a cam-shaft lpwhich simultaneously. cont1ols.=tlie;:coding.devices. One

camr-contact 2: interruptsthe .high tension supply to: the .valves.V2 V3. :-.during thesreturn. ssweep z-of the -.tuning condenser-sci .Another cam energises a IBIEYbIlOtIShOWIlUtO.01088 xthe :sWitch EA which" brings the condenser Cl 9 1' into parallel with the. condenser:-C-:for variable width. cod- .ing. An-extrawide; pulse may he radiatednas a distress signal and forEthispurpose.anotherrelay, notrshown, is energized which ;(a,)v closes a switch I to bring a condenser C2l-into parallelwiththe condenser C20, (b) closesra switcho ietoibring resistance Hi3 into-parallel with l a resistance R14 and (c). closes a switch-3 to. connect a condenser CM across the. cathode resistance R5. of the oscillator V3 in orderzto condition that circuit to the wider pulse. .Aselectorswitch allows the operator tocontrol the coding. as required.

The. supplementaryinterrogator required when identification signals. removed. in frequency ranges. from those used by .the radioelocation .equipment may conveniently comprise a. pushpull oscillator which is grid-modulated by-an impulse: derived .from themaster-control .of the radioelocation circuits, so. that it is locked .to-the time-base .of .the. main indicator. call is tuned .to a selected fspot? frequency.within theallotted wave=bandof 157-197 megacycles.

"The resulting. identification. signal from the responder is rendered visible .for interpretation on the screen oflthe cathode ray tube indicator When the responder is -installedin a: fixed beacon, it may. be desirableto send out theideni'lification signal ona different carrierfrequency from that used for the interrogating-signal;rso-

as to enablea pilot when takingobservations from the air toovercome the limitations imposed by ground-reflections, andcto'recognise a givenbeacon station at longer range thanwould otherwise be. possible. For this purpose, the-responder may The outgoing becoupled to two xdifierently-tuned. aerials,'1 one tant signals and discriminate in favour of nearer .3'1'0 'signalsaby the action .ofzappropriate:delayedauto- .maticgairrcontroh iIniorderato; allowxtherpilot toztake' :di'rectional observations,;and;..:if necessary, toassist heme. or: to tmakesa .blindelanding; the.:response signal may' :zconsist 1 01f. lapprolonged successionxof zn'arrowziorzwvide: pulses; vfollowed .byra: coded..sequence forJidentification purposes.

. :Another'; form. ofibeaconv responder.;for.1.provid:- .inglana'sligati'onal' .aidtto the pilot .of:'an interro gating; craft is'- arranged.'to .rtransmitxsignalston twoafrequencies, onexof iWhiChLiS identical .with the I call; whilst. the Otherzis si'slightly: different. The ffon: frequency response. serves. to give:-the

. .pil'ot1ofca machine :fitted with arsuitablerradiolocationizset... an indication: of; the position... and range'of :theinterrogatedbeacon; iwhilstithe dff .frequency response .can :be .usedpin combination with a switched aerial-system; to; provide anaapproach path of the overlapping-beam:type,:orcto assist blind-landing; The .fofi-frequency signals willalso .assist:the pilot to distinguish thebeacon response. from 1 other signal traceslduecto .refiec- 'tion..

.I claim:

1.;A :wireless signalling. system for establishing the:..identity of .a responder .in.tcommunication withlan. interrogator =Wherein.-. said .interrogator comprises .-a pulse-modulated radio transmitter .icrradiating a seriesof. separated radio-frequency pulses and .wherein. said responder comprisess. a

a radio: receiver,. a second; pulse-.modulated; radio transmitter, saidtransmitter :beingnormally held in; quiescent condition, circuit 1 means interconnecting saidreceiverawitha said second'transmitter :for ..:triggering the latter L .into. operatiomlto transmit .separatescounterpartc pulse .signals upon reception of 'pulsezsignalslbysaid receiver-,imeans for: altering: the individual. timeeduration .oi :the counterpart signal pulses transmitted by; saiclsece ondl. pulse=m0du1atedradio r.transmitterrfrom..one to anothersofi at least two. different andzprede- .termined values and cyclically. operated .coding means controlling. .said time-duration altering means .for causing acharacteristic and. repeated seriesfof changes of :timerdurati'on ofethe response pulses 'to :provide .an. identifying Iresponse asignal from the responder.

2. A a wireless 1 signalling system. according ato claim; 1 which mcludes discriminatoryselecting means'controll'ed by said cyclically operated zcod-v ing means 'for periodic'ally andrepeatedlw-inhibit ing-the efiectiveness of .thepulse signals .from sa-id first transmitter-upcnsaid receiver to causezitriggerinig-ofsaidsecond transmitter.

A 3. wireless signalling system according? to claim- 2 wherein-- said means for-*effectingraldiscriminating selection of --the interrogatin pulses comprises variable and cyclically operated :tuning meansfor said receiver.

t-A wireless signalling system. according to claim "3 :wherein said second transmitterdncludes variable. and cyclically operated tunings means, thearesonance frequency of said second transmit+ ter tuning. being held in stepwith that of:'said receiver tuning whereby-Ashecounterpart: pulse signals are radiated at substantia1ly th'elsame frequency as that of the interrogating pulse signa'ls' which caused operation of the triggering means.

551A pulse. echo system comprising in combinationa pulse-modulated radio transmitter; a radio receiver; cathode ray tube displaymeans assoe ciated with-said receiverfor' displaying pulse si nals received by: the latter in responseto the pulses of said transmitter and a pulse signal resaid second receiver, means associated with said second transmitter for altering the individual form of the response pulse signals generated by said second transmitter from one to another of at least two different and predetermined forms and cyclically operated coding means controlling said pulse form altering means for causing a characteristic and repeated series of changes of the form of the response pulse signals whereby the display of such response pulse signals on said display means of said first receiver provides identification of said responder.

6.,A pulse echo comprising in combination a pulse-modulated radio transmitter, a radio receiver, cathode ray tube displa means associated with said receiver for displaying the individual form of pulse signals received by the latter in response to the pulses of said transmitter and a pulse signal responder which comprises a second radio receiver, a second pulse-modulated radio transmitter normally held in quiescent condition, triggering means interconnected with said second receiver and said second transmitter for releasing said second transmitter to transmit a pulse signal in response to the reception of a pulse signal by said second receiver, means associated with said second transmitter for altering the individual form of the pulse signals generated by said second transmitter upon release by said triggering means from one to another of at least two different and predetermined forms and switching means controlling said pulse-form altering means for determining the displayed form of response pulses visible upon thecathode-raytube display means of said first radio receiver.

7. A pulse echo system comprising in combination a pulse-modulated radio transmitter, an associated radio receiver, cathode ray tube display means for displaying pulse signals received by said receiver in response to the pulses of said transmitter and a pulse signal responder which comprises a second radio receiver, a second pulsemodulated radio transmitter normally held in quiescent condition, triggering means interconnected with said second receiver and said second transmitter for releasing said second transmitter to transmit a pulse signa1 in response to the reception of a pulse signal by said second receiver, means associated with the pulse modulating circuits of said second transmitter for altering the time duration of the pulse signals generated by said second transmitter upon release by said triggering means from one to another of at least two different and predetermined values and'codin means controlling said means for altering the response pulse time duration, said coding means operating in cyclic manner to cause periodic changes the form of the displayed response pulses identifying said responder on said display means.

8. A wireless recognition signalling system comprising an interrogating station having a pulsemodulated radio transmitter, an associated pulse signal receiver and apparatus for displaying the form of pulse signals received by said receiver in response to the pulse signals radiated from said transmitter and a responder station comprising a second radio receiver, a second radio transmitter, having a pulse-modulating circuit, said second transmitter being normally held in quiescent condition, circuit means interconnecting said second radio receiver with said second transmitter for triggering the latter into operation to transmit a response pulse signal upon reception of a pulse signal by said second receiver, means associated with the pulse-modulating circuit of said second transmitter for providing at least two different characteristic individual forms of the signal pulse provided by said second transmitter upon each triggering thereof and cyclically operating coding means controlling said pulse form altering means to cause repeated changes in a characteristic cyclic manner of the individual form of the displayed response pulses on said apparatus of said first receiver.

9. A wireless recognition signalling system comprising an interrogating station having a pulse-modulated radio transmitter operating at a fixed chosen frequency, an associated pulse signal receiver and apparatus for displaying the form of pulse signals received by said receiver in response to the pulse signals radiated from said transmitter and a responder station comprising a second radio receiver, a second pulse-modulated radio transmitter, said transmitter being normally held in quiescent condition, cyclically operated means for varying the tuning of said second receiver and said second transmitter over a range of frequencies which includes that of said first transmitter, circuit means interconnecting said second radio receiver with said second transmitter for triggering the latter into operation to transmit a response pulse signal upon reception of :a pulse signal by said receiver and at substantially the same frequency as such received pulse signal and means for altering the individual form of the response pulse signal generated by said second transmitter upon each triggering thereof from one to another of at least two different and predetermined forms, said pulse form altering means being controlled by said cyclically operated tuning variation means for providing at said interrogating station a response signal display from said responder station which is an intermittent presentation of response pulse signals whose individually displayed form varies periodically.

10. A responder for use in a pulse-modulated wireless interrogation system comprising a radio receiver, a pulse-modulated radio transmitter, said transmitter being normally held in quiescent condition, circuit means interconnecting said receiver with said transmitter for causing operation of the latter to transmit a counterpart pulse signal in response to reception of a pulse signal by said receiver means for varying the individual time duration of each counterpart signal pulse from one to another of at least two different and predetermined values and switching means for controlling said means for altering the pulse time duration to provide at least two characteristically different forms of identifying response signal pulses from said responder.

11. A responder according to claim 10 which includes cyclically operated means for periodically and repeatedly inhibiting the effectiveness of pulse signals available at said receiver to cause triggering of said transmitter.

12. A responder according to claim 11 wherein said discriminating means comprises cyclically varying tuning means for said receiver.

13. A responder according to claim 12 in which said transmitter also includes cyclically varying tuning means, the resonance frequency of such transmitter tuning being held in step with that of the receiver tuning whereby the counterpart pulse signals are radiated at substantially the same frequency as that of the received pulse signals which caused them.

14. A responder for use in a pulse-modulated wireless interrogation system comprising a pulse signal receiver, a pulse-modulated radio transmitter, said transmitter being normally held in quiescent condition, circuit means interconnecting said radio receiver with said transmitter to cause operation of the latter to generate a counterpart pulse signal in response to the reception of a pulse signal by said receiver, means for altering the individual form of the counterpart signal pulses generated by said transmitter from one to another of at least two difierent and predetermined iorms, and cyclically operated coding means controlling said pulse form altering means for causing a characteristic and repeated series of changes of the form of the response pulses identifying the responder.

15. A responder according to claim 14 wherein said means for altering the individual form of said counterpart signal pulses includes means for providing the counterpart pulses with a time duration of a number of difierent values and switching means for selecting any one of said time duration values.

16. A responder according to claim 14 wherein said radio transmitter includes an oscillatory thermionic valve circuit which is normally suppressed by means of a blocking bias voltage and wherein said radio receiver is arranged to provide an output signal voltage suitable for reducing said blocking bias voltage to a value permitting operation of said normally suppressed oscillatory valve circuit.

17. A responder according to claim 14 wherein said radio receiver includes a super-regeneratively operated valve.

18. A responder according to claim 14 wherein said circuit means interconnecting said receiver with said radio transmitter includes a resistance capacity network the time constant of which serves to exert a control on the time duration of the radiated counterpart signal pulse.

19. A responder according to claim 14 wherein said radio receiver includes adjustable tuning means which are mechanically operated so as to be continuously varied in a cyclic manner over a predetermined tuning range.

20. A responder according to claim 14 wherein said radio receiver includes adjustable tuning means which are mechanically operated soas to be continuously varied in a cyclic manner over a predetermined tuning range and wherein said radio transmitter is also provided with adjustable tuning mechanism which is operated in step with that of the receiver tuning means whereby each transmitted counterpart signal is efiected at a frequency substantially identical with that of the received signal pulse which initiated it.

F'REDERIC CALLAND WILLIAMS.

REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,134,716 Gunn Nov. 1, 1938 2,159,937 Zworykin May 23, 1939 2,252,083 Luck Aug. 12, 1941 2,333,688 Shepard Nov. 9, 1943 2,415,667 Wheeler Feb. 11, 1947 2,416,346 Potter Feb. 25, 1947 2 ,418,139 Preisman Apr. 1, 1947 2,419,571 Labin et al. Apr. 29, 1947 2,421,016 Deloraine et al. May 27, 1947 2,453,970 Charrier Nov. 16, 1948 2,471,373 Joyner May 24, 1949 FOREIGN PATENTS Number Country Date 116,666 Australia Oct. 10, 1941 116,667 Australia Oct. 10, 1941 

